How can i optimize my query i have written to find the Users and there last order details using aggregate, it shows me timeout as the dataset is large - mongodb

Question

I have a query as below, what it does it creates a link between two documents and find the last order date and users details like email, phone, etc. but on large data set it shows me timeout error any help would be much appreciated, and thanks in advance for the help
db.users.aggregate([
{
"$lookup": {
"from": "orders",
"let": {
"id": "$_id"
},
"pipeline": [
{
"$addFields": {
"owner": {
"$toObjectId": "$owner"
}
}
},
{
"$match": {
$expr: {
$eq: [
"$owner",
"$$id"
]
}
}
},
],
"as": "orders"
}
},
{
"$unwind": {
path: "$orders",
preserveNullAndEmptyArrays: false,
includeArrayIndex: "arrayIndex"
}
},
{
"$group": {
"_id": "$_id",
"order": {
"$last": "$orders.createdAt"
},
"userInfo": {
"$mergeObjects": {
name: "$name",
email: "$email",
phone: "$phone",
orderCount: "$orderCount"
}
}
}
},
{
"$project": {
name: "$userInfo.name",
email: "$userInfo.email",
phone: "$userInfo.phone",
orderCount: "$userInfo.orderCount",
lastOrder: "$order",
}
}
]
)
my documents look like the following for orders
{
"_id": ObjectId("607fbeeb0a752a66a7af40eb"),
"address": {
"loc": [
-1,
3
],
"_id": "5d35d55d3d081f486d0d401c",
"apartment": "",
"description": "ACcdg dfef"
},
"approvedAt": ISODate("2021-04-21T11:28:05.295+05:30"),
"assignedAt": null,
"billingAddress": {
"description": ""
},
"createdAt": ISODate("2021-04-21T11:28:04.449+05:30"),
"creditCard": "",
"deliveryDate": "04/21/21",
"deliveryDateObj": ISODate("2021-04-21T12:27:58.746+05:30"),
"owner": "609bd5831b912947ea51a9ac",
"products": [
"5a070c079b"
],
"updatedAt": ISODate("2021-04-21T11:28:05.295+05:30"),
}
and for users, it is like below
{
"_id": ObjectId("609bd5831b912947ea51a9ac"),
"updatedAt": ISODate("2021-05-12T18:47:55.291+05:30"),
"createdAt": ISODate("2021-05-12T18:47:55.213+05:30"),
"email": "1012#gmail.com",
"phone": "123",
"dob": "1996-04-10",
"password": "",
"stripeID": "",
"__t": "Customer",
"name": {
"first": "A",
"last": "b"
},
"orderCount": 1,
"__v": 0,
"forgottenPassword": ""
}

Answer 1

convert _id to string in lookup's let and you can remove $addFields from lookup pipeline
add $project stage in lookup pipeline and show only required fields
$project to show required fields and get last / max createdAt date use $max, you don't need to $unwind and $group operation
db.users.aggregate([
{
$lookup: {
from: "orders",
let: { id: { $toString: "$_id" } },
pipeline: [
{ $match: { $expr: { $eq: ["$owner", "$$id"] } } },
{
$project: {
_id: 0,
createdAt: 1
}
}
],
"as": "orders"
}
},
{
$project: {
email: 1,
name: 1,
orderCount: { $size: "$orders" },
phone: 1,
lastOrder: { $max: "$orders.createdAt" }
}
}
])
Playground
SUGGESTION:
You can save owner id in orders as objectId instead of string and whenever new order arrive store it as objectId, you can prevent conversation operator $toString operation
create an index in owner field to make lookup process faster.

Answer 2

I have figured out that after using createIndex for the owner field which is used to compare the owner in the orders from the users _id filed, so just after adding an db.orders.createIndex({ owner: 1 }), the query will run much faster and smoother